The extraction of dissolved foreign matter from water is among the most important of industrial processes. Creating potable fresh water from sea water and the extraction and reclamation of dissolved industrial chemicals from waste water are two instances in which water distillation serves a vital economic and environmental role.
The most common method for distilling water entails a vapor cycle such that the condensate provides pure water while the dissolved foreign matter remains behind in the unvaporized source water. A less common alternative involves freezing, with the melt providing pure water and the remaining unfrozen source water retaining the foreign matter.
There are major thermodynamic advantages to the ice cycle. For example, it takes seven times as much heat to vaporize a given quantity of water as it takes to freeze it. Heat pump losses are therefore reduced in the ice cycle method by almost an order of magnitude. Other factors favoring the ice cycle include the elimination of concerns relating to vapor lock from gases dissolved in the source water, as well as reduced salt corrosion and scaling of the distilling apparatus.
A problem peculiar to the ice cycle is the tendency of impure source water to cling to the ice after it has been formed. It is customary with the ice cycle process, as in Korzonas U.S. Pat. No. 4,517,806, to use water that has already been distilled, or is otherwise sufficiently pure, to rinse the ice crystal pack before it is melted. This rinsing process--typically carried out in what may be called a wash column--has been identified as a major source of inefficiency since there is frequently a partial loss of ice due to a premature melt which is flushed along with the rinse water.
In principle, such losses in the rinse operation can be reduced and the purity of the product water improved by not immediately disposing of the fresh water which has been used to rinse the ice. Instead, this water could be reused in one or more rinse operations which precede the final rinse operation. The purity of the water product can theoretically be increased and the amount of rinse water required reduced to any desired value simply by incorporating a sufficient number of sequential rinse operations in which rinse water is reused. However, the additional mechanical complexity and energy requirement associated with each rinse, along with the melt losses incurred in each rinse, have discouraged such an approach.
In a typical prior art wash column such as disclosed in Martin U.S. Pat. No. 4,164,854 or the above mentioned patent to Korzonas, ice crystals are consolidated or packed together into a porous mass--which may be called an ice crystal pack, ice block or ice plug--by means of flotation pressure, in combination with the upward hydraulic pressure of the incoming salt water or ice/brine slurry. The hydraulic pressure also serves to propel the ice plug in an upward direction.
The density, porosity and draining characteristics of such an ice crystal pack, ice block or ice plug depend upon the particular conditions under which it is formed. This fact inevitably affects the efficiency of the resulting water purification. Thus, the unpredictable structure of the porous ice plug insures that the water will drain preferentially along those paths which offer the least resistance. Paths which offer more resistance are insufficiently rinsed, and in the extreme case there exist "bowls" or pockets of fused ice crystals which hold brine that is inaccessible to the rinse water.
When no rinse step is included, as in the above mentioned Martin patent, the brine is essentially squeezed out of the ice block formed by the compacting of a number of small ice crystals. Because of the unpredictable structure of the block formed according to this prior art teaching, there are again unavoidable deficiencies in this method of water purification.